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A front is a weather system that is the boundary separating two different types of air. One type of air is usually denser than the other, with different temperatures and different levels of humidity. This clashing of air types causes weather: rain, snow, cold days, hot days, and windy days.
The warm air mass rises as the cool air masses push and meet in the middle. The temperature drops as the warm air mass is occluded, or “cut off,” from the ground and pushed upward. Such fronts can bring strong winds and heavy precipitation. Occluded fronts usually form around mature low pressure areas.
Symbolically, an occluded front is represented by a solid line with alternating triangles and circles pointing the direction the front is moving. On colored weather maps, an occluded front is drawn with a solid purple line. The lower dew point temperatures behind the front indicate the presence of drier air.
The five major types of fronts (cold, warm, occluded, stationary and dry line phenomena) depend on the direction of the air mass’s travel and its characteristics.
warm front
Arctic air masses
Five air masses affect the United States during the course of a typical year: continental polar, continental arctic, continental tropical, maritime polar, and maritime tropical. Continental air masses are characterized by dry air near the surface while maritime air masses are moist.
Four major types of air masses influence the weather in North America:
They can bring anything from tropical warm and humid days to arctic cold depending on the type of air mass. Fronts form the boundaries of air masses with differing properties. The most severe weather usually occurs when dry-cold continental polar air clashes with warm-humid maritime tropical air.
Anticyclones are high-pressure centers of dry air. Theyare also called”highsi’Anticyclones lead to dry, clearweather. Because of the Coriolis effect, in the Northern Hemisphere winds spin in a counter- clockwise direction in a cyclone and in a clockrvise direction in an anticyclone.
An air mass is a large body of air with generally uniform temperature and humidity. The area over which an air mass originates is what provides its characteristics. The longer the air mass stays over its source region, the more likely it will acquire the properties of the surface below.
When two different air masses come into contact, they don’t mix. They push against each other along a line called a front. When a warm air mass meets a cold air mass, the warm air rises since it is lighter. At high altitude it cools, and the water vapor it contains condenses.
An air mass has roughly the same temperature and humidity. Air masses form over regions where the air is stable for a long enough time. The air takes on the characteristics of the region. Air masses move when they are pushed by high level winds.
Storms arise if the air mass and the region it moves over have different characteristics. For example, when a colder air mass moves over warmer ground, the bottom layer of air is heated. That air rises, forming clouds, rain, and sometimes thunderstorms.
What happens when air is heated or cooled? So air, like most other substances, expands when heated and contracts when cooled. Because there is more space between the molecules, the air is less dense than the surrounding matter and the hot air floats upward. This is the concept used in the hot air balloons.
Most tornadoes form from thunderstorms. You need warm, moist air from the Gulf of Mexico and cool, dry air from Canada. When these two air masses meet, they create instability in the atmosphere.
Arctic, Antarctic, and polar air masses are cold. The qualities of arctic air are developed over ice and snow-covered ground. Arctic air is deeply cold, colder than polar air masses. Arctic air can be shallow in the summer, and rapidly modify as it moves equatorward.
Air masses that form over the ocean, called maritime air masses, are more humid than those that form over land, called continental air masses.
But mA-type (maritime Arctic) does not exist. Continental Polar air masses form over large, high- latitude land masses, such as northern Canada or Siberia. cP air masses are cold and extremely dry. Wintertime cooling over these land areas cause the atmosphere to become very stable (even inversion).